ECOLOGY AND TAXONOMY OF Holimeda 229 



1. The ocean groups 



Table XIX groups the Halimeda species by region of the oceans in 

 which they have been found. Only seven of the modern species are 

 pantropical and are known at least for the western Atlantic, western 

 Pacific and eastern Indian Oceans. Only one of the pantropical species 

 has entered the Mediterranean Sea. Undoubtedly the lists for the 

 Indian, Pacific and Atlantic Oceans will increase with more collecting, 

 but definite patterns do emerge. Most striking of these is that character- 

 istic species swarms of Halimedae have evolved for the Atlantic and 

 Indo-Pacific regions, even though the environments of each are 

 sufficiently alike for seven species to be established throughout. The 

 youngest basin of warm water, the Mediterranean, has no endemic 

 species and only one of those found worldwide. This pattern seems to 

 speak of an ancient lineage in which local speciation proceeds but 

 slowly. 



Changes since the earlier revision of the genus (Hillis, 1959) include 

 the addition of new species and the extension of known distributions, 

 both of which are likely to occur in the future as well. It is particularly 

 likely that some of the species now known only for the Pacific may be 

 found in the Indian Ocean, where even less collecting has been done 

 than in other oceans. It would also be interesting to know if species 

 restricted to the Indian Ocean exist. 



Further collecting will also be needed to determine the true extent 

 of endemism within the genus, for it seems likely that the known ranges 

 of some very "local" species like cryptica, favulosa, gigas, goreauii, 

 lacrimosa, melanesica, scahra and velasquezii will be extended. 



Table XIX also shows that of the four regions with more than ten 

 Halimeda species, the greatest number is found in the Pacific, the least 

 in the Atlantic. This pattern is exhibited by other tropical organisms, 

 and may be a function both of area and of palaeohistory. This table also 

 indicates that more species occur in the western portions of the Pacific 

 and Atlantic Oceans. One might be tempted to conclude that the 

 Marshall Islands, Jamaica and the Bahamas, with their peak numbers 

 of Halimeda species for the Pacific and Atlantic Oceans respectively, 

 represent centres of speciation or, alternatively, that larger areas or 

 islands support more species of Halimeda. These conclusions, however, 

 are premature, for these and other high numbers mostly represent 

 regions of relatively intense collection and study. As many or possibly 

 more species of Halimeda may eventually be known for other sites when 

 their floras are adequately sampled and studied. The species list from 

 Honduras, for example, has increased from six to nine as a result of 



